It started with Vic Rail's horses, in September 1994. First one, then another, they died horrible deaths, 13 horses in all over the span of just two weeks, frothing from their noses and mouths, thrashing in agonizing pain. Then Rail died too.

Weeks later Australian officials isolated a newly discovered virus they ultimately named Hendra, after the Brisbane suburb where Rail and his horses died. For 17 years, Hendra virus smoldered in its host population of fruit bats killing nearly 50 horses and claiming three more human lives.

Then in May, something happened.

It was as if Hendra virus awoke from a slumber and roared fully into life. There have been more outbreaks of Hendra in 2011 – 18 at last count – than in the 16 previous years.

Veterinary epidemiologists hunting the virus now know definitively that Australia's fruit bats (Pteropus sp.), also called flying foxes, spread the disease to horses, which then can infect humans. And while they don't know the exact cause of the huge escalation in outbreaks, they strongly suspect it has something to do with the heavy rainfall and big floods that drowned northeastern Australia from November 2010 to February 2011.

And that has them looking nervously at climate change.

"The interesting change was the big floods in January," said Raina Plowright, a disease ecologist at the Pennsylvania State University's Center for Infectious Disease Dynamics. "Floods are expected more frequently with climate change – so, if they are linked, climate change may increase disease."

Hendra virus is just one of a number of newly emerged zoonotic diseases, so called because they have their origins in animals but somehow make the leap to humans, and in doing so, wreak enormous havoc. While zoonotic diseases may sound exotic, one of the most devastating is also one of the most familiar: AIDS, which made the jump from primates to humans sometime at the beginning of the 20th century, and now kills an estimated 2.7 million people a year. Hendra, far newer but fearsomely lethal, has claimed the lives of four of the seven people infected.

The alphabet soup of deadly and economically damaging zoonotic diseases is long and includes West Nile virus, avian influenza and SARS, or Severe Acute Respiratory Syndrome – another disease thought to have jumped from bats to humans.

World health officials concluded in 2004 that more than three-quarters of new, emerging or re-emerging human diseases today are caused by pathogens originating from animals or animal products. Overall, between 1940 and 2004, scientists estimate there have been more than 300 emerging disease events around the globe – a number that will likely grow as population grows.

Increased food production and animal husbandry of waterfowl and pigs, which can harbor viruses like influenza, help explain the increasing emergence of these new, often virulent diseases, said Jan Slingenbergh, senior animal health officer and head of the emergency prevention system for the Food and Agriculture Organization, or FAO, a United Nations program working for food security.

Climate change adds yet another complicating factor, Slingenbergh said. It can expand the range of insects and arthropods that can transmit disease. Or, as may be the case with Hendra, it can cause ecological upheaval, adding to the likelihood that people will come into contact with virus-carrying animals. Animals on the wing – birds and bats – are like sick people on airplanes, he added: they can travel long distances with their viral burden.

"Whether it is about Hendra, Nipah or Ebola or some other bat-circulating virus or non-human primate virus, we know that sooner or later they are going to show up as novel infectious diseases in humans," Slingenbergh said.

It's not possible to make a blanket statement about how climate change will affect outbreaks of zoonotic diseases, scientists say. The interplay between temperatures, rainfall and shifting habitats is too multifaceted to boil down into one overarching trend. Closer contact of wild and domestic animals as well as advanced detection technologies further complicate the picture.

But Linfa Wang, a scientist working on the Hendra outbreak for Australia's Commonwealth Scientific & Industrial Research Organization, or CSIRO, said climate change clearly amplifies other factors contributing to the increase in zoonotic outbreaks. "None of these virus outbreaks would have been identified 50 years ago," he said.

The world community has responded to this ever-increasing onslaught of diseases by looking harder for them before they become full-blown pandemics.

The U.S. Centers for Disease Control and Prevention runs the National Center for Zoonotic, Vector-Borne, and Enteric Diseases; part of its mandate is to evaluate how climate change will affect the prevalence and spread of zoonotic diseases, such as Hanta virus in the American Southwest.

CDC efforts aside, though, many see zoonotic diseases as confined to steamy jungles or crowded third-world marketplaces teeming with live chickens and pigs. Hendra's emergence, Plowright said, shows we need to forget that stereotype.

Think of it this way: Ebola, West Nile, Nipah, and now Hendra. If this list of virulent, deadly infectious agents were an SAT question that asked you to identify the one different from the others, what would be the answer? All four are named for locales where the "patient zero" case was isolated. All four are recently emerged zoonotic viruses.

But get out your atlas and look: Ebola is in the Democratic Republic of the Congo, West Nile is in East Africa, Nipah is in Malaysia – all but Hendra are named for places in underdeveloped countries. Hendra virus "is going on in a country just like America," Plowright said. "This is just like the prequel to Contagion."

Veterinarians believe one of the key causes of the Hendra virus outbreaks is environmental stress. Fruit bats naturally harbor the virus, but it doesn't make them physically ill. Instead, Plowright and others have found that when the bats are stressed, the percentage infected with Hendra rises dramatically, possibly making it easier for them to spread the disease to horses.

There are also paradoxical stresses, such as habitat encroachment, which send bats off on a hunt for new food sources. This brings them closer to people, because humans tend to plant fruit trees. Biologists have found that bats in some urban and suburban areas of Australia are abandoning their migratory behavior and becoming sedentary because neighborhood fruit trees are available. This sedentary behavior on the part of some animals has led populations to fragment, which may be another important reason behind the outbreaks.

Big populations of migratory bats mingle with one another, which epidemiologists believe allows the bats to share a low level of Hendra infection – conferring on the population as a whole something called herd immunity. But when normally migrating bats become sedentary, Hendra virus immunity may wane. "Our models predict that Hendra virus used to be like a slow burning fire with very little kindling, because it was everywhere," Plowright said. But now, it has "changed to a few big fires."

The call that really put veterinarians and public health officials on high alert came in July, when Dusty the dog, who had been living on a property in Mount Alford where three horses were infected with Hendra, also was found to have the disease. While the dog did not seem to be contagious, the finding opened another, more troubling route for human infections.

The virus also can lie dormant – Mark Preston, who helped perform an autopsy on a dead horse, died from the virus a year after he did the autopsy. And while Hendra is not now contagious in humans, when humans contract the disease, it has a lethality of close to 60 percent. Ebola, for comparison, has a fatality rate of 68 percent.

The outbreaks have also become part of the political landscape: There have been calls to remove Queensland Premier Anna Bligh and Agriculture Minister Tim Mulherin.

One thing is certain, health experts say: As the planet grows hotter and more crowded, the range and severity of zoonotic outbreaks will increase. Hendra, so far, has exacted a minor human toll. H5N1 avian flu, in contrast, required the culling of hundreds of millions of chickens, and cost Asia's farmers $10 billion.

World health officials, reading this trend, are trying to get ahead of it. They've issued a call to arms – "One Health," a global endeavor led by the FAO, the World Bank, the World Health Organization, the World Organisation for Animal Health and the United Nations Development Programme. More than 600 scientists and public health doctors are involved, pursuing the idea that the health of humans, animals and the environment are inextricably linked.

The current tack - where doctors treat people, veterinarians handle animals, and biologists focus on plants - doesn't work, said the FAO's Slingenbergh. "The microbes ... do not recognize our compartmentalized approach."

"An interdisciplinary approach is the only possible answer to stop this continuous generation of novel diseases."

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.

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ABOUT THE AUTHOR(S)

Nancy Bazilchuk is a freelance reporter based in Trondheim, Norway. Her work has appeared in Audubon, ScientificAmerican.com, and Conservation. DailyClimate.org is a foundation-funded news service that covers climate change.

Scientific American is part of Springer Nature, which owns or has commercial relations with thousands of scientific publications (many of them can be found at www.springernature.com/us). Scientific American maintains a strict policy of editorial independence in reporting developments in science to our readers.